Laser gyroscopes used for navigation often include mechanisms for individually dithering gyroscope mirrors or the entire gyroscope to avoid a problem in such systems known as "mode locking." Mode locking is the tendency of two counter rotating beams of light to appear to have the same frequency at low levels of angular velocity due to noise or light scattering, which results in a loss of the desired beat frequency produced by the laser gyroscope.
Although dithering solves the mode locking problem, it may create an additional problem when the dither frequencies are transmitted to the gyro case. Dithering the mirror or entire gyro causes vibrations which, if trnasmitted to the case, can interfere with the input or output of other instruments or gyros within a navigation system in which the first gyro is utilized. More particularly, when two or more of the gyros are mounted to a common base and are vibrating at close to the same frequencies, but not at identical frequencies, vibrations of one gyro can interfere with vibrations of the other gyros, either constructively, destructively, or both. In other words, the vibrational energy of one gyro can be transmitted to the others through the gyroscope mountings. This transmission of energy creates problems such as dither frequency lock-in, dither beating, and low dither depth which adversely affect the accuracy of the navigation system.
Although this interference can be reduced by providing an isolation arrangement within each gyro, it is not entirely eliminated. An example of an isolation arrangement may be found in U.S. Pat. No. 4,115,004 to Hutchings et al., assigned to the assignee of the present invention.
Further reduction of the interference can be provided by complex spring systems in the gyro hub such as the single spring system fround in U.S. Pat. No. 3,464,657 to Bullard which issued on Sept. 2, 1969, assigned to the United States of America, and the three-spring system taught in U.S. Pat. No. 4,309,107 to McNair, et al., entitled "Laser Gyroscope Dither Mechanism" which is assigned to the assignee of the present invention. Although the spring mounted gyro as taught in the McNair et al. patent substantially eliminates the mode locking problem, it is a complex arrangement which may not be necessary in some less demanding applications.
A less complex alternative for reducing the effect of vibrational interference by transmission of dither vibrations to the case is to tune the vibrational frequencies of the dithering gyros to either identical frequencies or frequencies of sufficiently different periods to avoid mechanical vibrational interference among them and hence minimize these problems.
Although dither frequencies of the individual gyros can be tuned electronically, this requires adding components to the gyroscope system. It has been found that by placing a portion of each gyroscope mounting structure in either tension or compression, the dithering for the separate gyroscopes can be effectively tuned to sufficiently different frequencies to eliminate the problems associated with mechanical interference of dither frequencies among the gyros.